1. Field of the Invention
Aspects of the present invention relate to an organic light emitting display and a fabricating method thereof, and more particularly, to an organic light emitting display and a fabricating method thereof capable of precisely controlling the position of crystallization of an amorphous silicon and forming an active layer (thin film transistor) by forming an alignment mark on a region of a substrate.
2. Description of the Related Art
Generally, an organic light emitting diode is an apparatus in which an electron and a hole are bound in a fluorescent and/or phosphorescent organic compound to thereby emit light by injecting the hole from an anode and injecting the electron from a cathode into a light emitting layer.
As shown in FIG. 1, an organic light emitting diode is comprises an anode electrode (ITO), which may be formed of Indium Tin Oxide, an organic thin film, and a cathode electrode (Metal). The organic thin film may comprise an emission layer (EML) to emit light by forming excitons as the electron and hole (+) bind to each other, an electron transport layer (ETL) to transport the electron, and a hole transport layer (HTL) to transport the hole (+). Also, an electron injecting layer (EIL) may be formed on one side of the electron transport layer (ETL), and a hole injecting layer (HIL) may further be formed on one side of the hole transport layer (HTL).
A passive matrix driving method and an active matrix driving method may be used to drive the organic light emitting diode. The passive matrix driving method has advantages that the manufacturing process is simple and the investment cost is low. The passive matrix driving method uses an anode and a cathode that cross in an orthogonal direction from which lines are selected and driven; however the passive matrix driving method is disadvantageous in that the current consumption is too high when used to drive a large size display. The active matrix driving method has advantages in that the current consumption is low, the display quality and life of the display is good, and the active matrix driving method may be applied to a medium to large size display by forming an active element and a storage capacitor at each pixel.
The fabricating method of the organic light emitting display includes an operation of crystallizing the amorphous silicon, an operation of fabricating the active layer (thin film transistor), and an operation of fabricating the organic light emitting diode. There is an encapsulation operation, a module assembly operation, and the like; however, the explanation of such operations will be omitted.
The operation of crystallizing the amorphous silicon comprises a substrate cleaning operation, a buffer layer forming operation, an amorphous silicon vapor depositing operation, a polysilicon forming operation, and the like.
The operation of fabricating the active layer comprises an operation of pattering the polysilicon, a gate insulating layer forming operation, a gate patterning operation, an ion injecting/activating operation, an interlayer dielectric layer forming operation, a contact forming operation, source/drain patterning operation, and the like. An insulating layer and via forming operation, an ITO forming operation, and a pixel definition layer forming operation are further executed.
The organic light emitting diode fabricating operation comprises a cleaning operation, a pretreatment operation, an organic light emitting diode vapor depositing operation, a cathode vapor depositing operation, and the like.
The forming positions of the amorphous silicon crystallizing operation and the active layer (thin film transistor) fabricating operation are precisely controlled so that a characteristic dispersion of the diode becomes minimal. For example, in the amorphous silicon crystallizing operation, it is preferable that the forming size and direction, etc. of the polysilicon are precisely controlled so that a grain boundary of the polysilicon has a maximum size and uniform directional property. In the active layer (thin film transistor) fabricating operation, it is preferable that the source/drain/gate are controlled and formed (patterned) so that they are positioned in the region and direction having an optimal grain boundary. Further, even in the gate electrode forming operation, it is preferable to form the gate electrode with respect to the size and direction of the polysilicon as described above.
However, the crystallization technology in consideration of position and direction during the crystallization process, which is converting the amorphous silicon to the polysilicon, and the technology in consideration of the position and direction during the active layer forming process are not well developed. The organic light emitting display, unlike a liquid crystal display, generally forms the active layer (thin film transistor) after the amorphous silicon is crystallized to form the polysilicon; however, any technology optimizing the position of the polysilicon and active layer (thin film transistor) is not well developed.
Therefore, the properties of the manufactured thin film transistor, such as threshold voltage, S-factor, off current, and mobility, are not uniform among the formed thin film transistors, and thus there is a problem that the luminance of the organic light emitting diode driven by the controlled current is also not uniform due to the lack of precision in forming the polysilicon and the active layer (thin film transistor).